1. Field of the Invention
The present invention relates to a method and apparatus for performing an efficient and reliable handover, for application to a variety of wireless access systems.
2. Discussion of the Background Art
A brief description will be made below of a conventional HandOver (HO) and ranging procedure.
HO is the process of maintaining a call in progress without interruption, when a Mobile Station (MS) moves from the cell area (or communication zone) of its serving Base Station (BS) to that of another BS during the call. In other words, HO is a function of ensuring the continuity of an on-going call by channel or link switching.
There are three types of HO: softer HO, soft HO, and hard HO.
Softer HO is an HO that takes place within the same cell. In softer HO, an MS switches to a better channel among channels in use within cell coverage.
Soft HO is an HO characterized by simultaneous use of two adjacent channels and gradual termination of one of the channels in the end. In Code Division Multiple Access (CDMA), soft HO is readily implemented due to use of the same frequency band.
Hard HO is a type of HO procedure in which an old channel is disconnected immediately before a new channel is established.
To move from a cell area of a serving BS to a cell area of a target BS and perform a handover to the target BS, the MS may perform a ranging procedure.
Ranging refers to a set of processes for maintaining the connection quality (especially synchronization) of Radio Frequency (RF) communication between the MS and the BS in Institute of Electrical and Electronics Engineers (IEEE) 802.16 or Mobile Internet. In multiple access based on Orthogonal Frequency Division Multiple Access/Time Division Duplexing (OFDMA/DD), ranging does not matter for the downlink because the BS transmits signals to a plurality of MSs at a single reference timing.
On the other hand, a plurality of MSs transmit signals individually on the uplink, resulting in different propagation delays. Therefore, ranging is carried out between the BS and an MS by a separate timing synchronization scheme for the uplink. That is, ranging is a procedure by which a plurality of MSs correct their transmission timings.
For the purpose of uplink synchronization acquisition and power control between the MS and the BS, initial ranging, periodic ranging, and HO ranging are defined. Also, Bandwidth Request (BR) ranging is defined for the MS to request a bandwidth to the BS.
Initial ranging occurs for an initial network entry and connection procedure, when the MS initially enters the BS, for example, when the MS is power-on or it transitions from a long sleep mode to an idle mode. Periodic ranging is performed periodically for the MS to track synchronization to the BS. To acquire synchronization with a target BS during handover, the MS performs HO ranging. BR ranging is used to request allocation of uplink resources to the BS. Only after acquiring system synchronization, the MS performs BR ranging.
Upon receipt of a ranging request message from the MS, the BS generates a Connection IDentifier (CID) for the MS. Then the BS performs re-negotiations, re-authorization, and re-registration, etc. with the MS.
A CID is an address that identifies a connection mapped to a required service flow between peers at a Medium Access Control (MAC) sublayer. The CID is included in a MAC Protocol Data Unit (PDU). That is, every connection between the BS and the MS is identified by a CID in a MAC header and the CID is mapped to a Service Flow IDentifier (SFID) that defines Quality of Service (QoS) parameters of the service flow associated with the CID.
Each MS may be allocated management CIDs from the BS during the MS's access to the BS, namely initialization. That is, the MS may be allocated management CIDs from the BS during a series of ranging and registration procedures. The management CIDs may be classified into a Basic CID (BCID), a Primary CID (CID), and a Secondary CID (SCID).
The BCID is used for the BS to identify the MS during an initial access. That is, the MAC address of the MS is mapped to the BCID. The PCID may be used for an authorization message and dynamic resource allocation-related messages. The SCID is optional and whether the SCID will be used may be determined during an MS capabilities negotiation.
Besides the management CIDs, there are a Transport CID (TCID) for identifying a service of user traffic, a broadcasting CID for identifying broadcast traffic, and a multicast CID. A TCID may be allocated to each connection established between the MS and the BS.
HandOver (HO) is a technology for enabling an MS to continue its on-going communication in a target BS by transitioning data contexts in which the MS was communicating with a serving BS to the target BS with a minimal interruption time, when the MS moves from the serving BS to the target BS. That is, HO ensures the continuity of a service that the MS is receiving.
Aside from normal handover, there are no specified procedures for uncontrolled handover. Moreover, during the uncontrolled handover, the MS should transmit its MAC address to the target BS to perform a ranging procedure. In this case, it may be difficult to guarantee location privacy for the MS.
That is, it is necessary to consider procedures for guaranteeing the location privacy of the MS to protect the MS against malicious security threats during the uncontrolled handover.
To provide location privacy to the MS, a hashed MAC address and a temporary STation ID (STID) may be used for the MS. However, since the hashed MAC address and/or the temporary STID should be allocated to the MS during ranging, the ranging procedure may be unnecessarily prolonged.